1. Field of the Invention
The present invention relates to a spread spectrum demodulator according to the direct sequence code division multiple access (DS-CDMA) scheme.
2. Description of Related Art
A DS-CDMA system consists of a spread spectrum modulator which spreads the bandwidth of a data signal by multiplying each of its symbols by a spreading code as a pseudo noise signal and transmits a resulting signal, and a spread spectrum demodulator which demodulates a reception signal through de-spreading in which the reception signal is multiplied by the same spreading code as used in the transmission side. In this DS-CDMA system, when a reception signal is de-spread in the spread spectrum demodulator, not only noise that was added on a transmission channel but also a transmission signal produced by using a different spreading code is de-spread. Therefore, a demodulated reception signal is scarcely influenced by such noise or a transmission signal. In general, a DC-CDMA system realizes multiplexing in the same frequency band by using different spreading codes for respective sets of a modulator and a demodulator that communicate with each other. However, even where the same spreading code is used for plural sets of a modulator and a demodulator, multiplexing is still possible because a transmission signal of a set other than the set concerned is spread if the symbol timing is not completely identical among the plural sets of a modulator and a demodulator.
FIG. 6A shows an example of a conventional spread spectrum modulation circuit 30 that is used in a DC-CDMA system, and FIG. 6B shows an example of a conventional spread spectrum demodulation circuit 31 that is used in a DC-CDMA system. In FIG. 6A, reference numeral 18 denotes a differential encoder for performing differential coding on transmission data 50; 6, a spreading code generator for generating a spreading code; 20, a spreading modulator for multiplying the transmission data that has been subjected to differential coding in the differential encoder 18 by the spreading code that has been generated by the spreading code generator 6; 21, a BPSK modulator for performing binary phase shift keying (BPSK) on the transmission data that has been spread by the spreading modulator 20; and 22, a transmission antenna for transmitting the transmission data that has been amplified.
In FIG. 6B, reference numeral 2 denotes a reception antenna for receiving data and numeral 3 denotes a BPSK demodulator for BPSK-demodulating the reception data. Reference symbols 4a and 4b denote A/D converters for converting demodulated orthogonal I-component data (I) and Q-component data (Q) into digital data. Reference symbols 5a and 5b denote correlators for demodulating, by de-spreading, the orthogonal digital I-component data (I) and Q-component data (Q) by multiplying those data by respective spreading codes generated by a spreading code generator 6 that are the same as used in the transmission side. Reference numeral 7 denotes a polar coordinates converter POLAR for polar-coordinates-converting the de-spread I-component data (I) and Q-component data (Q). Reference numeral 19 denotes an amplitude judgment device for performing cyclic addition and threshold judgment on amplitude-component data (r) that has been produced by the polar coordinates conversion. The amplitude judgment device 19 performs initial synchronization in which timing having the maximum correlation value among pieces of one-symbol timing is detected. Successively, reference numeral 9 denotes a Δf corrector for latching, with the timing that has been synchronization-confirmed by the amplitude judgment device 19, frequency-component data (φ) that has been produced by the polar coordinates conversion, and correcting for an offset of a carrier frequency of the spread spectrum modulator and demodulator. Reference numeral 10 denotes a delay detector for detecting the data that has been corrected by the Δf corrector 9.
The conventional spread spectrum demodulator 31 of FIG. 6B that is used in a DS-CDMA system can only cope with a case where one reception wave exists in a one-symbol timing. For example, if transmission waves that were spread by using the same spreading signal as used in the receiver side are transmitted from two transmitters simultaneously, two pieces of timing having almost the same correlation values that are larger than a threshold value are detected in a one-symbol timing. The receiver side judges that timing that happens to have the maximum value is reception timing. Therefore, which timing is acquired is indefinite, that is, depends on the correlation values that vary due to noise etc., resulting in a problem that a normal receiving operation cannot be performed.
Even if plural pieces of correct timing were detected (initial synchronization), the detection timing in the one-symbol timing that is predetermined by an operation clock signal of the receiver would gradually deviate due to errors between operation clock signals of the transmitter and receiver. Where synchronization is made with the same timing in each symbol timing, there arises a problem that errors occur in reception data.